1. Field of the Invention
The present invention relates to a switching regulator having a soft start circuit for suppressing a rush current for charging an output capacitor with electricity upon turn-ON of a power supply for the switching regulator.
2. Description of the Related Art
A general step-up switching regulator having a soft start circuit is configured in the form of a circuit as shown in FIG. 2.
An input power supply 8 is connected between one terminal of a coil 9 and GND. The other terminal of the coil 9 is connected to a drain of a switching transistor 7 and an anode of a rectifying diode 10. A cathode of the rectifying diode 10 is connected to one terminal of a voltage division resistor 3, one terminal of an output capacitor 11 and one terminal of a load 12. A voltage division output obtained through voltage division between the voltage division resistors 3 and 4 is connected to a minus terminal of an error amplifying circuit 5. An output of a reference voltage circuit 1 is connected to a plus terminal of the error amplifying circuit 5 through a soft start circuit 2. Also, an output of the error amplifying circuit 5 is connected to a gate of the switching transistor. 7 through a duty control circuit 6.
The error amplifying circuit 5 compares a voltage obtained by controlling a start time of the reference voltage using the soft start circuit 2 with a voltage obtained by dividing an output voltage using the voltage division resistors 3 and 4. An ON/OFF time of the switching transistor 7 is controlled with an output as the comparator results through a duty control circuit 6 so as to make the output voltage constant.
The function of the soft start circuit 2 is to cause the output from the reference voltage circuit 1 to rise slowly for a time period of several milliseconds, thereby preventing a rush current upon turn-ON of the power supply.
An example of a circuit configuration of a conventional soft start circuit is shown in FIG. 3.
A MOS transistor 13 is connected between an input terminal 21 and an output terminal 22. One of the terminals of each of a current source 15 and a capacitor 14 are connected to a gate of the MOS transistor 13. Since upon turn-ON of a power supply the current source 15 charges the capacitor 14 with electricity, a gate voltage of the MOS transistor 13 gradually rises. Thus, since an ON resistance of the MOS transistor 13 gradually decreases, a voltage inputted to the error amplifying circuit 5 slowly rises. In such a manner, while the output voltage upon turn-ON of the power supply is low, the reference voltage used for comparison is caused to gradually rise to realize the soft start (refer JP 10-271680 A for example).
However, the conventional soft start circuit necessarily acts on the output of the reference voltage circuit upon turn-ON of the power supply. Hence, there is encountered such a problem that even in an inspection process, the output voltage of the switching regulator does not become a desired voltage unless a time period required for the soft start elapses whenever the power supply is turned ON, which exerts an influence on the inspection time.